Farmall Electrical System

Operation:
Unlike other systems the switch must be H before the generator will charge. The switch also has a LO which is low charge but that never worked on my tractor.  Since it does not have a regulator I suspect you were to switch to low charge when you knew the battery was charged up.  You should be able to see the battery charging by a positive current flow on the amp meter.  The switch has 2 light settings Hi and Dim but Dim just inserts a resistor in series with the light bulbs and unless you have the original bulbs you may not notice any change.
 
Caution:
DO NOT run the lights without a battery because the system has no voltage regulator and the voltage from the generator will burn out the 6v light bulbs.

Wiring:
The generator wire connects to the reverse voltage cutout relay. A wire runs from the relay to the ON switch and then from the switch to the bat cable to charge the battery. Wires run from the switch back to the lights. (see diag)

Conversion:
There are basically 3 conversions. One involves replacing the generator and the starter with new 12v types. One only replaces the starter. The other only changes the battery.

The generator will generate a voltage high enough to charge a 12v battery (some say it won't but I think it depends on the generator).  Since the system does not have a regulator it only monitors charge current not voltage. You can install a 12v starter (if you can find one) or use the existing 6v one as long as don't run it for long times and overheat it,  it turns faster so the tractor starts better. You will need to install standard 12v light bulbs the sockets are normal bayonet with one point. Same as parking and backup lights. I found ones that look just like the original but were 12v at WallMart.
On mine I only changed the battery and the lights and it works fine. But I use a old battery with a dead cell  that would no longer work on my truck. Or I suggest a 12v Lawn Mower battery since it has lower current.

Most people are converting back to 6v nowadays.

This is my own modified wiring diagram.

Some tractors had a different generator that ran the lights without a battery and no starter. The same wiring harness was used and the F wire was not connected. The generator did not use a relay and had only 1 connection. I suspect that all of those have been converted.